Update ChainSetup and IdentityChain interface

[arandito]

Description

This PR refactors ChainSetup to store the resolved profile name instead of a parsed profile Section.

Downstream providers (e.g. assume-role) need ChainSetup to store the resolved profile name for cycle tracking. Storing the parsed Section alongside it would make the profile reachable two ways, setup.profile and setup.profile_file.get_profile(setup.profile_name), duplicating state. Using profile_name as the single source of truth lets providers read values consistently through MergedConfig.get(profile_name, key). That call also performs the string validation the previous _get_string helpers provided, so each provider no longer needs its own copy of that logic.

This PR also renames profile_file/profile_name_override to config_file/profile_name for consistency, and updates the EcsContainer module suggestion from aws-credentials-ecs to aws-credentials-http.

Testing

• Updated tests with new interfaces
• Ran make check-py and make test-py.

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[Alan4506]

This runs before ProfileAssumeRole and terminates assembly, so a profile with both static keys and role_arn resolves to the raw user keys and the role is never assumed. This breaks the standard self-referencing pattern, where the keys are only the source credentials for the STS call:

[profile deploy]
role_arn = arn:aws:iam::999999999999:role/Deployer
source_profile = deploy
aws_access_key_id = AKIA...          # near-zero-permission user, only sts:AssumeRole
aws_secret_access_key = ...          # real permissions live on the role

botocore gives role_arn precedence: botocore's assume-role provider runs before the static-keys providers and claims any profile containing role_arn, regardless of static keys (_has_assume_role_config_vars).

Let me know what you think.

[arandito]

Boto3's credential chain order is legacy and places assume role credentials directly after environment (docs).

However, modern SDKs must follow the order defined in the StandardProvider enum. This means that self-referencing profiles will not be respected if used in a chain.

They are still respected if you use the assume role resolver on its own.

[jonathan343]

LGTM